Networker media: (critical) Waiting for 2 writable volume(s) to backup pool

This depends on your configuration.

It could mean that you do not have any appendable backup volume labeled for this pool at all.

But you will also see this message if you try to backup more streams to that pool at the same time. This depends on your 'device parallelism' and the number of backup streams which you need to balance. However, without knowing more details, it is hard to tell you where to 'tune' your system.

May I suggest you read the appropriate chapters in the Admin and in in the Performance Tuning Guide for more details.

based on the pool name stating nwdd in its name, I'd say we are dealing with ddboost devices here? Possibly not enough sessions available to satisfy the amount of needed sessions of backusp running at the same time?

Not knowing how much backups you have running or what the parallelism is for the backup action(s) in question or the clients involved, if you have reach the amount of 60 max. sessions on the ddboost device, when the DD supports it to have more sessions, then one can share the same ddboost volume with another but even the same NW storage node.

What we do in large environments is having multiple pools, so separate ones for file system backups, and the different NMM, NMDA, NMSAP and even the archive log backups, so to have more devices an more sessions, but for the larger pools, we share the one and same volume with multiple ddboost devices on the same NW storage node. So we have gone up to 2, 3 or 4 times sharing the same boost volume, so one can end up with 4x60=240 simultaneous sessions to the same DD pool.

Also have a look at https://www.dell.com/support/kbdoc/en-us/000173177/networker-troubleshooting-media-waiting-events-waiting-for-1-writable-volume-or-no-matching-devices

and the referenced https://supportkb.dell.com/attachment/ka06P000000LYVBQA4/Troubleshoot_Media_Events_csp_en_US_1.pdf (is from 2016 still can help)